The present invention relates generally to nonlinear acoustic systems that utilize the non-linearity of a propagation medium for signal demodulation, and more specifically to a method of processing signals in a nonlinear acoustic system to reduce distortion in resulting demodulated signals.
Nonlinear acoustic systems are known that employ an acoustic transducer for projecting an ultrasonic carrier signal modulated with a processed audio signal through the air for subsequent regeneration of the audio signal along a path of projection. Such nonlinear acoustic systems typically include a modulator for modulating an ultrasonic carrier signal with a processed audio signal, a driver amplifier for amplifying the modulated carrier signal, and at least one acoustic transducer for directing the ultrasonic signal through the air along a selected projection path. Because of the nonlinear propagation characteristics of the air, the projected ultrasonic signal is demodulated as it passes through the air, thereby regenerating the audio signal along the selected projection path.
One drawback of typical nonlinear acoustic systems is that the regenerated audio signals frequently contain significant levels of distortion.
An approach to reducing distortion levels in such regenerated audio signals is described in a publication entitled Parametric Loudspeaker—Characteristics of Acoustic Field and Suitable Modulation of Carrier Ultrasound, Aoki et al., Electronics and Communications in Japan, Part 3, Vol. 74, No. 9, 1991. According to that publication, an audible signal level generated by the nonlinear acoustic process is approximately proportional to the square of the modulation envelope for low levels of the ultrasonic signal, and approximately proportional to the modulation envelope itself for high levels of the ultrasonic signal. In order to invert the distortion that would normally result in the audible signal, Aoki et al. employ a processing method that combines taking the square root of the audio signal and multiplying the audio signal by an empirically determined constant before modulation.
Although the approach of Aoki et al. reduces distortion for specific ultrasonic output levels, this approach has drawbacks in that it generally does not reduce distortion over a full output level range of the ultrasonic signal.
It would therefore be desirable to have a nonlinear acoustic system that can be used to regenerate audible signals with reduced distortion. Such a system would reduce distortion in regenerated audio signals over a full practical range of ultrasonic output levels.